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Abstract:

There is provided an acrylic adhesive composition which comprises: (A) a
vinyl polymer comprising a homopolymer or a copolymer of an alkyl
(meth)acrylate having a C4 to C14 alkyl group; (B) an organophilic
layered double hydroxide organically treated with an organic anion; and
(C) a crosslinking agent. Therefore the obtained adhesive composition has
sufficient adhesiveness to an object, excellent heat resistance and
excellent adhesive properties particularly at higher temperatures, and is
substantially free from variations in adhesiveness.

Claims:

1. An acrylic adhesive composition comprising: (A) a vinyl polymer
comprising a homopolymer or a copolymer of an alkyl (meth)acrylate having
a C4 to C14 alkyl group; (B) an organophilic layered double
hydroxide organically treated with an organic anion; and (C) a
crosslinking agent.

2. An acrylic adhesive composition as set forth in claim 1, wherein the
organic anion for the component (B) is at least one of an anion derived
from an amino acid, and an anion derived from a fatty acid.

3. An acrylic adhesive composition as set forth in claim 1, wherein a
layered double hydroxide to be organically treated to provide the
component (B) is a non-stoichiometric compound represented by one of the
following general formulae (1) and (2):
[M2+1-xM3+x(OH)2]x+[An-.sub.x/n.mH.sub-
.2O]x- (1)
[Li.sup.+1-xM3+x(OH)2].sup.(2x-1)+[An-.sub.(2x-1-
)/n.mH2O].sup.(2x-1)- (2) wherein M2+ is a metal ion of at
least one divalent metal selected from the group consisting of Mg, Ca,
Mn, Fe, Co, Ni, Cu and Zn; M3+ is a metal ion of at least one
trivalent metal selected from the group consisting of Al, Fe, Cr, Mn, Co,
Ni, La and Ga; An- is an n-valent inorganic anion selected from the
group consisting of OH.sup.-, ClO.sub.3.sup.-, ClO.sub.4.sup.-, F.sup.-,
Cl.sup.-, Br.sup.-, I.sup.-, CO.sub.3.sup.2-, NO.sub.3.sup.- and
SO.sub.4.sup.2-; x is a positive number with 0<x<0.5; m is a
positive number with 0<m; and n is a valence of the inorganic anion.

4. An acrylic adhesive composition as set forth in claim 1, wherein the
component (B) is present in a proportion of not less than 1 part by
weight and less than 20 parts by weight based on 100 parts by weight of
the component (A).

5. An acrylic adhesive composition as set forth in claim 1, wherein the
component (A) is a copolymer of butyl acrylate and acrylic acid.

6. An acrylic adhesive composition as set forth in claim 1, wherein the
component (C) crosslinks molecular chains of the vinyl polymer (A), and
is at least one selected from the group consisting of a multifunctional
(meth)acrylate having at least two (meth)acryloyl functional groups in
its molecule, an isocyanate compound and an epoxy compound.

8. An acrylic adhesive composition as set forth in claim 2, wherein a
layered double hydroxide to be organically treated to provide the
component (B) is a non-stoichiometric compound represented by one of the
following general formulae (1) and (2):
[M2+1-xM3+x(OH)2]x+[An-.sub.x/n.mH.sub-
.2O]x- (1)
[Li.sup.+1-xM3+x(OH)2].sup.(2x-1)+[An-.sub.(2x-1-
)/n.mH2O].sup.(2x-1)- (2) wherein M2+ is a metal ion of at
least one divalent metal selected from the group consisting of Mg, Ca,
Mn, Fe, Co, Ni, Cu and Zn; M3+ is a metal ion of at least one
trivalent metal selected from the group consisting of Al, Fe, Cr, Mn, Co,
Ni, La and Ga; An- is an n-valent inorganic anion selected from the
group consisting of OH.sup.-, ClO.sub.3.sup.-, ClO.sub.4.sup.-, F.sup.-,
Cl.sup.-, Br.sup.-, I.sup.-, CO.sub.3.sup.2-, NO.sub.3.sup.- and
SO.sub.4.sup.2-; x is a positive number with 0<x<0.5; m is a
positive number with 0<m; and n is a valence of the inorganic anion.

Description:

TECHNICAL FIELD

[0001] The present invention relates to an acrylic adhesive composition,
and a pressure-sensitive adhesive sheet employing the same. More
specifically, the invention relates to an acrylic adhesive composition
which has sufficient adhesiveness to an object, excellent heat resistance
and excellent adhesive properties particularly at higher temperatures,
and is substantially free from variations in adhesiveness, and to a
pressure-sensitive adhesive sheet employing the adhesive composition.

BACKGROUND ART

[0002] In recent years, adhesive tapes have come to be employed in a wide
range of applications, e.g., for production and construction of
electronic parts and automotive parts. In these applications, the
adhesive tapes are often subjected to higher stresses and higher
temperatures during use and, therefore, required to be highly cohesive
and heat-resistant. Particularly, adhesive tapes to be used in production
processes for electronic parts, semiconductor devices and flat display
devices such as LCDs and PDPs are generally subjected to temperatures of
100° C. or higher and, therefore, required to have sufficient
adhesiveness and cohesiveness at higher temperatures.

[0003] However, prior-art adhesive compositions are disadvantageously
poorer in heat resistance and cohesiveness at higher temperatures. One
approach to this problem is to blend various types of inorganic fillers
to an adhesive agent for improvement of the cohesiveness and the heat
resistance. Particularly, it is reported that, where a lipophilic layered
clay mineral (hereinafter referred to simply as "layered clay mineral")
is dispersed as the inorganic filler in the adhesive agent, the heat
resistance and the cohesiveness are both improved (see Patent Documents 1
and 2). [0004] Patent Document 1: JP-A-2005-344008 [0005] Patent Document
2: JP-A-2005-154581

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

[0006] However, even if an organophilic layered clay mineral prepared by
organically modifying a layered clay mineral (e.g., a smectite such as
montmorillonite, beidellite, nontronite, saponite, hectorite or
stevensite, or a mica such as muscovite, phlogopite, taeniolite, biotite,
margarite, clintonite or tetrasilisic mica) with an alkyl ammonium salt
is dispersed in the adhesive agent, the organophilic layered clay mineral
does not undergo complete interlayer separation, but is present in an
adhesive agent layer with its layers being in a stacked state. This
results in variations in adhesiveness at higher temperatures.

[0007] In view of the foregoing, it is an object of the present invention
to provide an acrylic adhesive composition which has sufficient
adhesiveness to an object, excellent heat resistance and excellent
adhesive properties particularly at higher temperatures, and is
substantially free from variations in adhesiveness, and to provide a
pressure-sensitive adhesive sheet employing the adhesive composition.

Means for Solving the Problems

[0008] According to a first aspect of the present invention to achieve the
aforementioned object, there is provided an acrylic adhesive composition
which comprises: (A) a vinyl polymer comprising a homopolymer or a
copolymer of an alkyl (meth)acrylate having a C4 to C14 alkyl
group; (B) an organophilic layered double hydroxide organically treated
with an organic anion; and (C) a crosslinking agent.

[0009] According to a second aspect of the present invention, there is
provided a pressure-sensitive adhesive sheet comprising the acrylic
adhesive composition according to the first inventive aspect.

[0010] Inventors of the present invention conducted intensive studies on
an inorganic filler and a resin composition in order to provide an
adhesive material having sufficient adhesiveness and sufficient heat
resistance. As a result, the inventors found that a layered clay mineral
dispersed in an adhesive agent does not undergo complete interlayer
separation, but the layered clay mineral is dispersed in the adhesive
agent with its layers being in a stacked state. Therefore, the resulting
adhesive composition suffers from variations in physical properties.
Further, the inventors found that, where an adhesive tape is produced by
using the adhesive composition, stress is liable to be concentrated on
peripheries of particles of the layered clay mineral in the adhesive
agent during peel-off of the adhesive tape, resulting in cohesive failure
of the adhesive agent. As a result of further studies, the inventors
found that, where an organophilic layered double hydroxide (B)
organically treated with an organic anion is used in combination with a
specific vinyl polymer (A) and a crosslinking agent (C) as described
above, a pressure-sensitive adhesive sheet can be provided which has
sufficient adhesiveness to an object, excellent heat resistance and
excellent adhesive properties particularly at higher temperatures and is
substantially free from variations in adhesiveness, and attained the
present invention. The physical properties of the adhesive composition
are improved with little variations supposedly because the organic
treatment with the organic anion promotes the interlayer separation of
the layered double hydroxide, and the use of the organophilic layered
double hydroxide in combination with the specific vinyl polymer and the
crosslinking agent further disintegrates the layered double hydroxide in
the adhesive agent and improves the dispersibility.

EFFECTS OF THE INVENTION

[0011] As described above, the present invention provides an acrylic
adhesive composition which comprises a vinyl polymer (A) comprising a
homopolymer or copolymer of a specific alkyl (meth)acrylate, an
organophilic layered double hydroxide (B) organically treated with an
organic anion, and a crosslinking agent (C). Such adhesive composition
has sufficient adhesiveness to an object, excellent heat resistance and
excellent adhesive properties particularly at higher temperatures, and is
substantially free from variations in adhesiveness. Accordingly, a
pressure-sensitive adhesive sheet comprising the adhesive composition as
an adhesive agent has the same advantageous effects.

[0012] Where the organic anion for the component (B) is at least one of an
anion derived from an amino acid, such as an amino acid anion, and an
anion derived from a fatty acid, such as a fatty acid anion, the
dispersibility of the layered double hydroxide in the adhesive agent is
improved, thereby suppressing variations in physical properties.

[0013] Where a layered double hydroxide to be organically treated to
provide the component (B) is a non-stoichiometric compound represented by
one of the following general formulae (1) and (2), the adhesive
composition is superior in adhesiveness and heat resistance.

wherein M2+ is a metal ion of at least one divalent metal selected
from the group consisting of Mg, Ca, Mn, Fe, Co, Ni, Cu and Zn; M3+
is a metal ion of at least one trivalent metal selected from the group
consisting of Al, Fe, Cr, Mn, Co, Ni, La and Ga; An- is an n-valent
inorganic anion selected from the group consisting of OH.sup.-,
ClO3.sup.-, ClO4.sup.-, F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-,
CO32-, NO3.sup.- and SO42-; x is a positive
number of 0<x<0.5; m is a positive number of 0<m; and n is a
valence of the inorganic anion.

[0014] Where the proportion of the component (B) is not less than 1 part
by weight and less than 20 parts by weight based on 100 parts by weight
of the component (A), the adhesive composition is free from variations in
adhesiveness, and improved in cohesiveness and heat resistance without
impairment of fluidity and formability.

[0015] Where the component (A) is a copolymer of butyl acrylate and
acrylic acid, the adhesive composition is further excellent in
adhesiveness and cohesiveness.

[0016] Where the component (C) crosslinks molecular chains of the vinyl
polymer (A) and is at least one selected from the group consisting of a
multifunctional (meth)acrylate having at least two (meth)acryloyl
functional groups in its molecule, an isocyanate compound and an epoxy
compound, the adhesive composition is further excellent in cohesiveness
and heat resistance.

[0018] As previously described, the vinyl polymer (A) is a homopolymer or
a copolymer of an alkyl (meth)acrylate having a C4 to C14 alkyl
group. The homopolymer or the copolymer are prepared by polymerizing a
polymerizable monomer containing an alkyl (meth)acrylate having a C4
to C14 alkyl group as a major component. In the present invention,
the (meth)acrylate is defined as at least one of an acrylate and a
methacrylate.

[0019] As described above, the polymerizable monomer includes the alkyl
(meth)acrylate having the C4 to C14 alkyl group as the major
component. Examples of an alkyl (meth)acrylate having the C4 to
C14 alkyl group (hereinafter sometimes referred to simply as "alkyl
(meth)acrylate") include acrylates and methacrylates each having a
C4 to C14 alkyl group such as a butyl group, an isobutyl group,
a pentyl group, an isopentyl group, a hexyl group, a butyl group, an
octyl group, an isooctyl group, a nonyl group, an isononyl group, a decyl
group or an isodecyl group. These may be used either alone or in
combination as the major polymerizable monomer.

[0020] As the polymerizable monomer, other copolymerizable monomer may be
used in combination with the alkyl (meth)acrylate. Particularly, a
polar-group-containing monomer is preferably used in combination with the
alkyl (meth)acrylate.

[0021] Examples of the polar-group-containing monomer include unsaturated
acids such as (meth)acrylate, itaconic acid, 2-acrylamidopropanesulfonic
acid, and hydroxyl-containing monomers such as 2-hydroxyethyl
(meth)acrylate and 2-hydroxypropyl (meth)acrylate, which may be used
either alone or in combination.

[0022] Here, "the alkyl (meth)acrylate as the major component" means that
the alkyl (meth)acrylate accounts for a major percentage of the entire
polymerizable monomer, and means that the polymerizable monomer may
contain the alkyl (meth)acrylate alone (to provide the homopolymer).

[0023] Where the alkyl (meth)acrylate and the polar-group-containing
monomer are used in combination, the proportion of the alkyl
(meth)acrylate is preferably 85 to 97 wt %, more preferably 90 to 95 wt
%, based on the weight of the polymerizable monomer, and the proportion
of the polar-group-containing monomer is preferably 15 to 3 wt %, more
preferably 10 to 5 wt %, based on the weight of the polymerizable
monomer. If the proportion of the latter is greater than the upper limit
described above, the resulting adhesive composition tends to have higher
adhesiveness and lower breaking elongation, failing to have advantageous
properties. On the other hand, if the proportion of the latter is less
than the lower limit described above, the resulting adhesive composition
tends to be poorer in adhesiveness and cohesiveness.

Organophilic Layered Double Hydroxide (B)

[0024] The organophilic layered double hydroxide (B) to be used in
combination with the vinyl polymer (A) is prepared by organically
treating an untreated layered double hydroxide (subjected to no organic
treatment) with the organic anion. Typically usable as the organophilic
layered double hydroxide (B) is a layered double hydroxide prepared by
ion-exchanging exchangeable anions (inorganic anions) present between
layers of the untreated layered double hydroxide with organic anions.
First, the untreated layered double hydroxide and then the organic
treatment will be described.

Untreated Layered Double Hydroxide

[0025] The untreated layered double hydroxide has a layered structure such
that hydroxide layers composed of hydroxides containing metal ions of a
monovalent or divalent metal and metal ions of a trivalent metal and
inorganic anion intermediate layers are alternately stacked, and water
molecules are intercalated between the layers (water is condensed between
the layers). The layered double hydroxide typically has a crystalline
structure. An example of the monovalent metal is Li. Examples of the
divalent metal include Mg, Ca, Mn, Fe, Co, Ni, Cu and Zn. Examples of the
trivalent metal include Al, Fe, Cr, Mn, Co, Ni, La and Ga. The hydroxide
layers each include oxygen octahedrons two-dimensionally arranged with
metal ions of the monovalent or divalent metal and the metal ions of the
trivalent metal located at centers of the oxygen octahedrons.

[0026] The untreated layered double hydroxide is known as so-called
"hydrotalcite" which is a natural mineral represented by
Mg6Al2(OH)16.CO3.4-5H2O. A variety of minerals,
such as Stichtite, Pyroaurite, Reevesite, Takovite, Honessite and
Iowaite, having the same or similar crystalline structure have been
discovered. These minerals are synthesizable. These minerals are
generally called "hydrotalcite-like" compounds, which are represented,
for example, by the following general formula (1) or (2):

wherein M2+ is a metal ion of at least one divalent metal selected
from the group consisting of Mg, Ca, Mn, Fe, Co, Ni, Cu and Zn; M3+
is a metal ion of at least one trivalent metal selected from the group
consisting of Al, Fe, Cr, Mn, Co, Ni, La and Ga; An- is an n-valent
inorganic anion selected from the group consisting of OH.sup.-,
ClO3.sup.-, ClO4.sup.-, F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-,
CO32-, NO3.sup.- and SO42-; x is a positive
number of 0<x<0.5; m is a positive number of 0<m; and n is a
valence of the inorganic anion.

[0027] The compound of the divalent-trivalent system (containing the
divalent metal ion and the trivalent metal ion in combination)
represented by the general formula (1) is a non-stoichiometric compound
(0<x<0.5), and can be synthesized as having different combinations
of the divalent metal ion and the trivalent metal ion and different
composition ratios.

[0028] The crystalline structure of the compound is as follows. Basic
layers [M2+1-xM3+x(OH)2]x+, which are
similar to those of Brucite (Mg(OH)2) and have positive charges, are
produced by replacing some of divalent metal ions (M2+) with
trivalent metal ions (M3+). Then, inorganic anion intermediate
layers ([An-x/n.mH2O]x-) having negative charges are
formed to electrically neutralize the basic layers. Thus, a layered
structure is formed which includes the basic layers and the inorganic
anion intermediate layers. In the layered structure, the water molecules
are typically hydrogen-bonded with hydroxide groups in the basic layers,
and maintained in association with the anion intermediate layers.

[0029] In Carlos J. Sema, Jose L. Rendon, Juan E. Iglesias,
"Crystal-chemical study of layered
[Al2Li(OH)6]PLX(super-).nH2O," Clays and Clay
Minerals, June, 1982, 30, 180-184, it is reported that the compound of
the monovalent-trivalent system (containing the monovalent metal ion and
the trivalent metal ion in combination) represented by the general
formula (2) provides a layered double hydroxide having a crystalline
structure similar to that described above. That is, trivalent metal ions
(e.g., Al ions) are arranged in a Gibbsite structure, and vacancies in
the structure are filled with monovalent metal ions (e.g., Li ions) to
provide two-dimensional layers, which are electrically neutralized with
anions intercalated between the layers.

[0030] The untreated layered double hydroxide (LDH) to be used in the
present invention is generally intended to include Hydrotalcite and
Hydrotalcite-like compounds to be described below.

[0031] The Hydrotalcite and the Hydrotalcite-like compounds each have a
structural unit including positively charged basic layers, inorganic
anion intermediate layers which electrically neutralize the positive
charges of the basic layers, and crystal water. It is known that these
compounds have substantially the same properties except for structural
failure temperature. These compounds are basic in solid state and
anion-exchangeable, and undergo specific reactions such as an
intercalation reaction and a reconstruction reaction. These compounds are
described in detail in Miyata Shigeo, "Properties and Applications of
Hydrotalcites," Smectite Newsletter, Smectite, Vol. 6, No. 1, p. 12-26,
1996 May.

[0032] In the present invention, the untreated layered double hydroxide is
organically treated with the organic anion in order to improve the
affinity for the vinyl polymer (A). Next, the organically treated layered
double hydroxide will be described.

Organically Treated Layered Double Hydroxide

[0033] The organically treated layered double hydroxide (hereinafter
referred to simply as "organophilic layered double hydroxide") to be used
in the present invention is a layered double hydroxide prepared by
ion-exchanging exchangeable anions (inorganic anions) present between the
layers of the untreated layered double hydroxide with organic anions.

[0034] Preferred examples of the organic anions to be used for the organic
treatment include organic anions derived from an amino acid, a fatty acid
and the like.

[0035] Examples of the organic anion derived from the amino acid include
amino acid anions and peptide anions. More specifically, these anions may
be prepared by using a neutral amino acid, a basic amino acid, an amino
acid having at least two acidic groups, a salt (an alkali metal salt such
as a sodium salt or a potassium salt, an alkali earth metal salt such as
a magnesium salt or a calcium salt) of the amino acid, or a peptide.
These may be used either alone or in combination.

[0037] Examples of the basic amino acid include lysine (Lys),
hydroxylysine (Hyl), arginine (Arg), histidine (His) and ornithine, which
may be used either alone or in combination. Among these basic amino
acids, lysine (Lys), hydroxylysine (Hyl) and arginine (Arg) are
preferably used.

[0039] Among these amino acids, aspartic acid (Asp), glutamic acid (Glu),
nitrilotriacetic acid (NTA), 1,2-diaminocyclohexane-N,N-tetraacetic acid
(DCyTA), ethylenediamine-N,N-diacetic acid and ethylenediaminetetraacetic
acid (EDTA) are particularly preferred. In the present invention, the
amino acid is defined in a broader sense as including an L-amino acid and
D-amino acid, and including not only an α-amino acid but also a
β-amino acid and a γ-amino acid.

[0040] Other examples include: amino acid derivatives such as
6-aminohexylcarboxylic acid, 12-aminolaurylcarboxylic acid,
N,N-dimethyl-6-aminohexylcarboxylic acid,
N-n-dodecyl-N,N-dimethyl-10-aminodecylcarboxylic acid and
dimethyl-N-12-aminolaurylcarboxylic acid; sulfur-containing compounds
such as 2-chlorobenzothiazole, thioacetic acid, methyldithiocarbamic acid
and dimethyldithianocarbamic acid, and salts thereof; and
nitrogen-containing heterocyclic compounds such as 2-mercaptothiazoline,
2,5-dimercapto-1,3,4-thiadiazole, 1-carboxymethyl-5-mercapto-1H-tetrazole
and 2,4,6-trimercapto-s-triazine and salts thereof. These may be used
either alone or in combination.

wherein R1-- is a C1 to C24 alkyl group, a C2 to
C24 alkenyl group, or an unsubstituted or an alkyl-substituted
phenyl group.

[0042] The organic anion may be an anion of a monosulfonic acid
represented by the following general formula (4), and specific examples
of the monosulfonic acid include dodecylsulfuric acid and
dodecylbenzenesulfuric acid.

R2--SO3H (4)

wherein R2-- is a C1 to C24 alkyl group, a C2 to
C24 alkenyl group, or an unsubstituted or an alkyl-substituted
phenyl group.

[0043] Further, the organic anion may be an anion of a dicarboxylic acid
represented by the following general formula (5), and specific examples
of the dicarboxylic acid include dibasic acids such as oxalic acid,
maleic acid, thiodipropionic acid and dithiopropionic acid, esters of any
of these dibasic acids with a substituted or unsubstituted aliphatic,
alicyclic or aromatic alcohol, and cyclic organic acids such as benzoic
acid, methylbenzoic acid, butylbenzoic acid, p-t-butylbenzoic acid,
phenylacetic acid, salicylic acid, fumaric acid, naphthoic acid, abietic
acid, phenylstearic acid, hydrinecarboxylic acid, cinnamic acid, rhodinic
acid and naphthenic acid, which may be used either alone or in
combination.

HOOC--R3--COOH (5)

wherein R3-- is a C1 to C24 alkyl group, a C2 to
C24 alkenyl group or an unsubstituted or an alkyl-substituted
phenylene group.

[0044] Next, the method of ion exchange for organic modification of the
present invention will be explained.

Ion Exchange for Organic Modification

[0045] The untreated layered double hydroxide intrinsically includes
interlayer anions, which are ion-exchangeable. In the present invention,
the exchangeable anions are ion-exchanged with the organic anions in
order to increase the affinity for the vinyl polymer (A) and to promote
interlayer separation of the layered double hydroxide. For the ion
exchange, the intended anions (organic anions) are intercalated into the
layered double hydroxide through direct ion exchange in an aqueous
solution or through ion exchange by a reconstruction method. Ions having
a higher charge density are more easily intercalated for the ion
exchange, so that anions each having a higher valence and a smaller ion
radius are more easily intercalated between the layers. Therefore, a
layered double hydroxide containing monovalent anions such as chloride
ions or nitrate ions between layers thereof is preferably used as a
precursor for the ion exchange.

[0046] For the direct ion exchange in the aqueous solution, the layered
double hydroxide containing the interlayer monovalent anions is generally
used as a host for the intercalation. The layered double hydroxide is
preferably synthesized in a nitrogen gas atmosphere or by bubbling
nitrogen gas in order to minimize contamination with carbonate ions from
the air. Since the layered double hydroxide specifically has affinity for
the carbonate ions, the layered double hydroxide is liable to be
converted into a layered double hydroxide of a carbonate ion type with
most anions thereof exchanged with the carbonate ions, failing to undergo
the ion exchange with the intended anions (organic anions).

[0047] The layered double hydroxide can be reconstructed by immersing a
pyrolysis product of the layered double hydroxide in an aqueous solution.
By utilizing this nature rather than by the direct ion exchange in the
aqueous solution, the intended anions (organic anions) can be
intercalated into the layered double hydroxide (reconstruction method).
That is, when the pyrolysis product of the layered double hydroxide is
immersed in the aqueous solution, anions present in the aqueous solution
are intercalated into the pyrolysis product to reconstruct the layered
double hydroxide. Therefore, the intercalation of the organic anions
between the layers can be achieved during the reconstruction by
preliminarily feeding the intended anions (organic anions) in the aqueous
solution.

[0048] For preparation of the pyrolysis product of the layered double
hydroxide, the layered double hydroxide is preferably fired at a heating
temperature of 400° C. to 800° C. If the heating
temperature is higher than the higher limit described above (e.g.,
900° C. or higher in the case of an Mg--Al layered double
hydroxide), spinel is liable to be separated from the layered double
hydroxide, making it difficult to reconstruct the layered double
hydroxide. On the other hand, if the heating temperature is lower than
the lower limit described above, the pyrolysis is insufficient, and the
carbonate ions are liable to remain.

[0049] Thus, the organophilic layered double hydroxide (B) according to
the present invention is prepared. The proportion of the organophilic
layered double hydroxide (B) is preferably not less than 1 part and less
than 20 parts by weight (hereinafter referred to simply as "parts") based
on 100 parts of the vinyl polymer (A). If the proportion is less than the
lower limit described above, intended effects cannot be produced for the
heat resistance and the cohesiveness. On the other hand, if the
proportion is not less than the upper limit described above, the
organophilic layered double hydroxide tends to be unevenly dispersed in
the adhesive agent.

[0050] The dimensions of the organophilic layered double hydroxide (B)
vary depending upon the chemical composition and the degree of the
crystal growth. Typically, the basic unit layers each have a thickness (a
single layer thickness) of about 0.47 nm, which may slightly vary
depending upon the metal elements for the layers. The crystalline
particles of the organophilic layered double hydroxide are in a plate
crystal form, and have an average length of 0.02 to 10 μm, preferably
0.02 to 7 μm, particularly preferably 0.1 to 5 μm. The average
length of the particles is determined by directly observing the particles
by means of an electron microscope (TEM or SEM) or the like. If the
average length of the layered double hydroxide particles is less than the
lower limit described above, the particles are not properly oriented in
the vinyl polymer, failing to provide sufficient cohesiveness and
adhesiveness. On the other hand, if the average length is greater than
the upper limit described above, the layers strongly interact with each
other. Therefore, uniform interlayer separation and dispersion of the
organophilic layered double hydroxide is difficult, thereby reducing the
elongation.

[0051] Exemplary methods for dispersing the organophilic layered double
hydroxide (B) in the resin composition include an ultrasonic separation
method, a high pressure shearing separation method, a very high speed
stirring method, a supercritical CO2 stirring method and the like.
Particularly, the high pressure shearing separation method is preferably
used, because this method achieves the interlayer separation without
fracture of the layered double hydroxide.

Crosslinking Agent (C)

[0052] In the present invention, the crosslinking agent (C) is used in
combination with the components (A) and (B). The crosslinking agent (C)
crosslinks molecular chains of the vinyl polymer (A) to increase the
adhesiveness and the cohesiveness of the pressure-sensitive adhesive
composition for improvement of the workability. Examples of the
crosslinking agent (C) include multifunctional (meth)acrylates each
having two or more (meth)acryloyl functional groups in a molecule thereof
(hereinafter referred to simply as "multifunctional (meth)acrylates"),
isocyanate compounds and epoxy compounds. The (meth)acryloyl group herein
means at lest one of an acryloyl group and a methacryloyl group, and the
(meth)acrylate herein means at least one of an acrylate and a
methacrylate.

[0053] Examples of the multifunctional (meth)acrylates include
trimethylolpropane tri(meth)acrylate, pentaerythritol
tetra(meth)acrylate, 1,2-ethylene glycol di(meth)acrylate, and
1,6-hexanediol di(meth)acrylate, which may be used either alone or in
combination.

[0054] Examples of the isocyanate compounds include: lower aliphatic
polyisocyanates such as butylene diisocyanate and hexamethylene
diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate,
cyclohexylene diisocyanate and isophorone diisocyanate; aromatic
isocyanates such as 2,4-tolylene diisocyanate, 4,4'-diphenylmethane
diisocyanate and xylylene diisocyanate; and isocyanate adducts such as a
trimethylolpropane/tolylene diisocyanate trimer adduct (CORONATE L
available from Nippon Polyurethane Co., Ltd.) and a
trimethylolpropane/hexamethylene diisocyanate trimer adduct (CORONATE HL
available from Nippon Polyurethane Co., Ltd.). These may be used either
alone or in combination.

[0055] Examples of the epoxy compounds include
N,N,N',N'-tetraglycidyl-m-xylenediamine (TETRAD-X available from
Mitsubishi Gas Chemical Company, Inc.) and
1,3-bis(N,N-diglycidylaminomethyl)cyclohexane (TETRAD-C available from
Mitsubishi Gas Chemical Company, Inc.). These may be used either alone or
in combination.

[0056] The proportion of the crosslinking agent (C) is 0.02 to 5 parts,
preferably 0.1 to 3 parts based on 100 parts of the vinyl polymer (A).
Where the crosslinking agent is difunctional, the proportion is higher
within this range. Where the crosslinking agent is trifunctional or more,
the proportion is lower within this range. If the proportion is less than
the lower limit described above, the crosslinking agent fails to produce
its effect. On the other hand, if the proportion is greater than the
upper limit described above, the resulting adhesive composition tends to
be fragile. In either case, the workability tends to be impaired.

[0057] As required, various known additives such as an anti-aging agent
and a colorant may be blended in the inventive adhesive composition which
contains the components (A) to (C).

Pressure-Sensitive Adhesive Sheet

[0058] The inventive pressure-sensitive adhesive sheet is produced, for
example, by forming the aforementioned adhesive composition into a sheet
or a tape, or by forming an adhesive layer of the aforementioned
composition on one or both of opposite surfaces of a base sheet.

[0059] The inventive adhesive sheet thus produced includes an adhesive
layer of the inventive adhesive composition preferably having a thickness
of 2 to 50 μm.

[0060] Examples of the base sheet to be used for the pressure-sensitive
adhesive sheet include a polyethylene terephthalate (PET) film, a
polyethylene naphthalate (PEN) film, a polyethersulfone (PES) film, a
polyetherimide (PEI) film, a polysulfone (PSF) film, a
polyphenylenesulfide (PPS) film, a polyetheretherketone (PEEK) film, a
polyarylate (PAR) film, an aramide film, a polyimide film and a liquid
crystal polymer (LCP) film. Particularly, the polyimide material is
preferred for heat resistance. The thickness of the base sheet is not
particularly limited, but preferably about 10 to 250 μm irrespective
of the type of the sheet or the film.

[0061] In the present invention, a protective film may be used for
protecting the adhesive layer and preventing adhesion to other parts.
Examples of the protective film include plastic films such as of
silicones, long-chain alkyl polymers, fluorinated polymers, aliphatic
amide polymers, and polyvinyl chlorides, vinyl chloride copolymers,
polyethylene terephthalates, polybutylene terephthalates, polyurethanes,
ethylene vinyl acetate copolymers, ionomer resins, ethylene-(meth)acrylic
acid copolymers, ethylene-(meth)acrylate copolymers, polystyrenes and
polycarbonates which are treated with a silica release agent to be
imparted with releasability.

[0062] Films of polyolefin resins such as polyethylenes, polypropylenes,
polybutenes, polybutadienes and polymethylpentenes intrinsically have
releasability even without treatment with the release agent and,
therefore, are each usable as the protective film. The protective film
preferably has a thickness of about 10 to about 100 μm.

[0063] The inventive adhesive composition and the inventive adhesive sheet
employing the adhesive composition can be produced, for example, by any
of the following methods (i) to (iii):

(i) A solution of the vinyl polymer (A) is prepared by solution
polymerization of the polymerizable monomer, and then the organophilic
layered double hydroxide (B) and the crosslinking agent (C) and,
optionally, other additives are added to the vinyl polymer solution.
Thus, a solution of the inventive adhesive composition is prepared, which
is applied onto a releasable base sheet or a non-releasable base sheet,
and a solvent is evaporated away by drying to provide the adhesive sheet.
(ii) A solid vinyl polymer (A) containing no solvent is prepared by
solution polymerization, suspension polymerization or mass polymerization
(UV polymerization) of the polymerizable monomer, and then the
organophilic layered double hydroxide (B) and the crosslinking agent (C)
and, optionally, other additives are added to the vinyl polymer. Thus,
the inventive adhesive composition is prepared, which is extruded by a
melt-extruder to provide the adhesive sheet. (iii) The organophilic
layered double hydroxide (B) and the crosslinking agent (C) and,
optionally, other additives are mixed with the polymerizable monomer, and
the resulting mixture is applied onto the base sheet. Then, a transparent
mask such as of a polyester film is placed on the applied mixture, which
is in turn irradiated with ultraviolet radiation to be thereby
polymerized. Thus, the inventive adhesive composition containing a matrix
of the vinyl polymer (A) and the inventive adhesive sheet are produced.

[0064] In the methods (i) to (iii), a proper polymerization initiator may
be used for the polymerization of the polymerizable monomer. As the
polymerization initiator, exemplary thermal polymerization initiators
which thermally generate radicals include azo compounds such as
azobisisobutyronitrile, and organic peroxides such as benzoyl peroxide
and cumene hydroperoxide. Exemplary photo polymerization initiators which
generate radicals by photoaction include benzoin ethers, substituted
benzoin ethers, substituted acetophenones, substituted α-ketols,
aromatic sulfonyl chlorides and photoactive oximes.

[0065] The applications of the inventive pressure-sensitive adhesive sheet
are not particularly limited, but exemplary applications include
production and construction of electronic parts and automotive parts,
which require heat resistance and cohesiveness.

[0066] Next, inventive examples will be described in conjunction with
comparative examples. It should be understood that the invention be not
limited to the inventive examples.

EXAMPLES

[0067] Preparation and synthesis of the following ingredients will be
described prior to the description of the examples.

Vinyl Polymer (A)

[0068] First, 100 parts of butyl acrylate, 3 parts of acrylic acid and 150
parts of toluene were put in a reaction vessel provided with a condenser,
a nitrogen introduction tube, a thermometer and a stirrer, and the
resulting mixture was stirred at 60° C. for 1 hour with nitrogen
gas being introduced into the vessel. Then, 0.2 parts of benzoyl peroxide
was added to the mixture, and a reaction was allowed to proceed at
60° C. for 10 hours. Thus, an acryl polymer was prepared.

[0070] First, 16.2 g of magnesium chloride hexahydrate
(MgCl2.6H2O of a special grade reagent available from Wako Pure
Chemical Industries, Ltd.) and 10.0 g of aluminum chloride hexahydrate
(AlCl3.6H2O of a special grade reagent available from Wako Pure
Chemical Industries, Ltd.) were dissolved in 4 L of decarbonated
distilled water, whereby an aqueous solution (MgCl2/AlCl3)
containing magnesium chloride and aluminum chloride in a blend ratio of
Mg/Al=2/1 (ratio in mol %) was prepared. Then, 16.9 g of sodium hydroxide
(NaOH of a special grade reagent available from Wako Pure Chemical
Industries, Ltd.) was dissolved in 1 L of decarbonated distilled water,
whereby a sodium hydroxide aqueous solution was prepared. The sodium
hydroxide aqueous solution was added dropwise to the
MgCl2/AlCl3 aqueous solution to adjust the final pH at about
10, while the MgCl2/AlCl3 aqueous solution was stirred. After
the resulting solution was aged at about 60° C. for 6 hours in a
nitrogen atmosphere, the resulting product was repeatedly filtered and
rinsed, and dried. Thus, a powdery layered double hydroxide A was
prepared.

Synthesis 2: Layered Double Hydroxide B

[0071] A layered double hydroxide B was prepared in substantially the same
manner as in Synthesis 1, except that an MgCl2/AlCl3 aqueous
solution containing 31.0 g of magnesium chloride hexahydrate and 10.0 g
of aluminum chloride hexahydrate in a ratio of Mg/Al=4/1 (ratio in mol %)
was used instead of the MgCl2/AlCl3 aqueous solution of
Synthesis 1.

Synthesis 3: Layered Double Hydroxide C

[0072] A layered double hydroxide C was prepared in substantially the same
manner as in Synthesis 1, except that a ZnCl2/AlCl3 aqueous
solution containing 11.3 g of zinc chloride (ZnCl2 of a special
grade reagent available from Kanto Chemical Industry Co., Ltd.) and 10.0
g of aluminum chloride hexahydrate in a ratio of Zn/Al=2/1 (ratio in mol
%) was used instead of the MgCl2/AlCl3 aqueous solution of
Synthesis 1.

Synthesis 4: Layered Double Hydroxide D

[0073] A layered double hydroxide D was prepared in substantially the same
manner as in Synthesis 1, except that a CoCl2/AlCl3 aqueous
solution containing 19.7 g of cobalt chloride hexahydrate
(CoCl2.6H2O of a special grade reagent available from Wako Pure
Chemical Industries, Ltd.) and 10.0 g of aluminum chloride hexahydrate in
a ratio of Co/Al=2/1 (ratio in mol %) was used instead of the
MgCl2/AlCl3 aqueous solution of Synthesis 1.

Synthesis 5: Layered Double Hydroxide E

[0074] A layered double hydroxide E was prepared in substantially the same
manner as in Synthesis 1, except that an NiCl2/AlCl3 aqueous
solution containing 19.7 g of nickel chloride hexahydrate
(NiCl2.6H2O of a special grade reagent available from Wako Pure
Chemical Industries, Ltd.) and 9.7 g of aluminum chloride hexahydrate in
a ratio of Ni/Al=2/1 (ratio in mol %) was used instead of the
MgCl2/AlCl3 aqueous solution of Synthesis 1.

Synthesis 6: Layered Double Hydroxide F

[0075] A layered double hydroxide F was prepared in substantially the same
manner as in Synthesis 1, except that a CaCl2/AlCl3 aqueous
solution containing 8.9 g of calcium chloride (CaCl2 of a special
grade reagent available from Wako Pure Chemical Industries, Ltd.) and 9.7
g of aluminum chloride hexahydrate in a ratio of Ca/Al=2/1 (ratio in mol
%) was used instead of the MgCl2/AlCl3 aqueous solution of
Synthesis 1.

Synthesis 7: Layered Double Hydroxide G

[0076] First, 9.6 g of lithium hydroxide (LiOH of a special grade reagent
available from Kanto Chemical Industry Co., Ltd.) was dissolved in 200 mL
of distilled water, whereby a lithium hydroxide aqueous solution was
prepared. Then, aluminum nitrate nonahydrate was dissolved in
decarbonated distilled water to prepare an aluminum nitrate aqueous
solution having a concentration of 0.5 mol/L. The aluminum nitrate
aqueous solution was slowly added dropwise to the lithium hydroxide
aqueous solution to adjust the final pH at about 10 to about 11, while
the lithium hydroxide aqueous solution was stirred. Thus, precipitate was
provided. In this state, the precipitate was aged at about 60° C.
for three days, repeatedly filtered and rinsed, and dried. Thus, a
powdery layered double hydroxide G was prepared.

[0077] The basal spacing of each of the layered double hydroxides thus
prepared was measured by the following test method. The results of the
measurement are shown in Table 1. Here, the basal spacing is herein
defined as a distance between the basal plane of the first layer and the
basal plane of the second adjacent layer, and corresponds to a distance
between periodic structures which appear as peaks in an X-ray diffraction
pattern.

Measurement of Basal Spacing

[0078] The basal spacing of each of the synthesized layered double
hydroxides was determined through X-ray diffraction (XRD) measurement.
With the use of an X-ray diffraction apparatus (RINT2200 available from
Rigaku Corporation), the measurement was performed with CuKα
radiation at 40 kV and 30 mA, a divergence slit angle of 1 degree, a
divergence vertical limit slit width of 10 mm, a scattering slit width of
1.25 mm, a receiving slit width of 0.3 mm, a scan speed of 2 degrees/min
and a sampling width of 0.02 degrees.

[0079] Next, the layered double hydroxides prepared in the aforementioned
manner were organically treated in the following manner.

Organic Treatment of Layered Double Hydroxides

Synthesis 8: Organophilic Layered Double Hydroxide a

[0080] First, 6.5 g of the layered double hydroxide A prepared in
Synthesis 1 was put in 1 L of decarbonated distilled water, and the
resulting mixture was stirred at 40° C. with nitrogen gas being
bubbled. Thus, a homogeneous dispersion was prepared. Then, 3.54 g of
sodium laurate (available from Wako Pure Chemical Industries, Ltd.) was
dissolved in 300 mL of decarbonated distilled water at about 50°
C., and the resulting solution was added as an organic treatment agent to
the dispersion. The resulting mixture was stirred at about 50° C.
for 3 hours. Thereafter, the resulting product was filtered and rinsed
three times, and finally dried and triturated. Thus, a organophilic
layered double hydroxide a was prepared.

Synthesis 9: Organophilic Layered Double Hydroxide b

[0081] An organophilic layered double hydroxide b was prepared in
substantially the same manner as in Synthesis 8, except that the layered
double hydroxide B was used instead of the layered double hydroxide A of
Synthesis 8.

Synthesis 10: Organophilic Layered Double Hydroxide c

[0082] An organophilic layered double hydroxide c was prepared in
substantially the same manner as in Synthesis 8, except that the layered
double hydroxide C was used instead of the layered double hydroxide A of
Synthesis 8.

Synthesis 11: Organophilic Layered Double Hydroxide d

[0083] An organophilic layered double hydroxide d was prepared in
substantially the same manner as in Synthesis 8, except that the layered
double hydroxide D was used instead of the layered double hydroxide A of
Synthesis 8.

Synthesis 12: Organophilic Layered Double Hydroxide e

[0084] An organophilic layered double hydroxide e was prepared in
substantially the same manner as in Synthesis 8, except that the layered
double hydroxide E was used instead of the layered double hydroxide A of
Synthesis 8.

Synthesis 13: Organophilic Layered Double Hydroxide f

[0085] An organophilic layered double hydroxide f was prepared in
substantially the same manner as in Synthesis 8, except that the layered
double hydroxide F was used instead of the layered double hydroxide A of
Synthesis 8.

Synthesis 14: Organophilic Layered Double Hydroxide g

[0086] An organophilic layered double hydroxide g was prepared in
substantially the same manner as in Synthesis 8, except that the layered
double hydroxide G was used instead of the layered double hydroxide A of
Synthesis 8.

[0087] The basal spacing of each of the organophilic layered double
hydroxides thus prepared was measured in the aforementioned manner. The
results are shown in Table 2.

[0088] As apparent from the results shown in Table 2, the organophilic
layered double hydroxides prepared in Syntheses 8 to 14 each have an
increased basal spacing, indicating that lauric acid molecules were
intercalated between layers of the organophilic layered double hydroxide.

[0092] The aforementioned ingredients to be used in the examples and the
comparative examples were blended in proportions shown in Table 3 to
prepare composite adhesive solutions. More specifically, acryl adhesive
solutions were each prepared by mixing the vinyl polymer (A) and the
crosslinking agent (C) in proportions shown in Table 3. Then, the
synthesized organophilic layered double hydroxides (B) and the layered
clay mineral were respectively blended in the acryl adhesive solutions,
and the resulting mixtures are each stirred by a stirrer and further
stirred by a wet ultra-fine disintegrator (NANOMIZER (registered trade
mark) available from Yoshida Kikai Kogyo Co., Ltd.) Thus, Composite
adhesive solutions were prepared.

[0093] The composite adhesive solutions thus prepared were each applied
onto a 25-μm thick polyimide film (APICAL 25NPI available from Kaneka
Corporation), and dried to form a 23-μm thick adhesive layer. A
polyester film (MRF50 available from Mitsubishi Chemical Polyester Film
Co., Ltd.) having a surface treated with a silicone release agent was
applied on the adhesive layer with the treated surface thereof in contact
with the adhesive layer. Thus, a pressure-sensitive adhesive sheet was
produced. For use, the polyester film treated with the silicone release
agent is removed from the pressure-sensitive adhesive sheet, and bonded
to an object.

[0094] The pressure-sensitive adhesive sheet thus produced was brought
into intimate contact with a glass plate, and an adhesive strength test
was performed in the following manner by a 180-degree peeling method. The
results are shown in Table 4.

Adhesive Strength

[0095] The adhesive strength test was performed in conformity with JIS
20237 by using a soda glass plate as an object to be bonded. A test piece
having a size of 20 mm (width)×80 mm (length) was cut out of the
pressure-sensitive adhesive sheet, and pressed on the glass test plate at
23° C. at 60% RH by moving a 2-kg rubber roller back and forth
over the test piece. The test piece was allowed to stand for 30 minutes,
and set in a tensile tester. Then, an adhesive strength was determined by
peeling the test piece at 180 degrees at a tensile speed of 300 mm/min,
and defined as "adhesive strength before heating." Another test piece of
the adhesive sheet applied on a glass plate in the same manner as
described above was heated at 150° C. for 1 hour, and cooled to a
room temperature. Then, an adhesive strength was determined in the same
manner, and defined as "adhesive strength after heating."

[0096] The results shown in Table 4 indicate that the adhesive sheets of
the inventive examples had sufficient adhesiveness to the object and,
even after the heating, had excellent adhesive properties. Further, the
adhesive sheets of the inventive examples each had little variations in
adhesive strength. Where a homopolymer of butyl acrylate, or a
homopolymer or a copolymer of other (meth)acrylate containing a C4
to C14 alkyl group was used instead of the vinyl polymer (A) in the
inventive examples, the same effects as described above were provided.

[0097] On the other hand, the adhesive sheets of Comparative Examples 1
and 2 which contained no organophilic layered double hydroxide (B) were
poorer in adhesive properties after heating and in heat resistance. The
adhesive sheet of Comparative Example 3 which contained the organophilic
layered clay mineral (synthetic hydrophobic smectite) was further poorer
in adhesive properties after heating and in heat resistance.

INDUSTRIAL APPLICABILITY

[0098] The inventive acrylic adhesive composition is used as a
pressure-sensitive adhesive sheet material which requires heat resistance
and cohesiveness in production and construction of electronic parts and
automotive parts.